Protection of electric distribution systems



P. v, HUNTER AND I. R. BEARD.

PROTECTION OF ELECTRIC DISTRIBUTION SYSTEMS.

APPLICATION FILED MAY 6. I9I8.

1,339,394. Patented May 11, 1920.

PILOT WIRE CONDUCTOR WW I TL j vvvvv v- CONDUCTOR UNITED STATES PATENT OFFICE.

IHILIP VASSARI-IUNTER, OFPURLEY, AND JAMES ROBERT BEAR-D, OF BENTON,

' ENGLAND.

PROTECTION OF ELECTRIC DISTRIBUTION SYSTEMS.

Specification of Letters Patent.

Patented May 11, 1920.

Application filed May 6, 1918. Serial No. 232,984.

T 0 all whom it may concern Be it known that we, PHILIP VAssAn HUNTER and JAMES Ronnn'r BEARD, subjects of the King of England, and residing, respectively, at Purley, Surrey, in England, and Benton, county of Northumberland, in England, have invented certain new and useful Improvements in the Protection of Electric Distribution Systems, of which the following is a specification.

This invention relates to the protection of electric distribution systems in which current transformers connected by pilot wires are employed.

In alternating current distribution systems wherein the feeder, transformer, or other apparatus is protected by a relay connected in circuit with pilot wires a difiicuty sometimes arises through the capacity current of the pilot wire which flows through the relay, causing the latter to operate on overload although the feeder may not be faulty. This difliculty arises especially when the pilot wire is of great length. It has been sought to overcome this drawback by employing compensating windings on the relays but it is believed that such a plan has not proved practical.

According to the present invention the difficulty is overcome by providing the pilot.

wire with a special conductor which may be termed a compensating conductor which is severed or open circuited so that no current can flow through it from end to end but is so connected and disposed with relation to the pilot wire that the relays, tripping coils or other protective devices are unaffected by the capacity current.

In one arrangement according to this invention the core of the pilot cable is provided with a compensating conductor in the form of a conducting shield surrounding both the core and its insulation. may be insulated and both it and the pilot core are connected to form a circuit for capacity current.

The shield is open circuited or severed and theoretically the point of such open circuiting should be adjacent to the middle of the pilot wire to obtain the most sensitive arrangement, but it would usually be permissible in this arrangement to make such open circuiting point anywhere along the length of the shield without appreciably affecting the sensitiveness and in practice it The shield would probably be, for convenience, near or at one end or the other of the pilot wire.

In an alternative arrangement the compensating conductor is in the form of a core corresponding to the pilot core and forming a companion conductor thereto so that the two form a pair carrying equal amounts of capacity current. In this arrangement the compensating conductor is severed or open circuited at or near the point giving equality of capacity current carried by the compensating and pilot conductors.

In the accompanying drawings which illustrate diagrammatically two protective arrangements according to this invention Figure 1 illustrates an arrangement wherein the compensating conductor is in the form of a shield.

Fig. 2 is a section of a pilot cable for use therewith.

Fig. 3' shows an alternative arrangement i which the compensating conductor is a companion core to the pilot core, and

Fig. 4: is a section of a pilot cable suit able for use with the arrangement illustrated in Fig. 3.

lVith reference first to Figs. 1 and 2, A, A represent relays and B, B the secondaries of current transformers. C is the pilot core and D, D the compensating conductor in the form of a shield shown diagrammatically as a line in Fig. 1 but actually tubular and surrounding the pilot core C as indi cated in Fig. 2. In Fig. l the shield D, D is shown as open circuited at a Joint adja cent to the middle of the core although as previously mentioned it is not essential, in this arrangement, that the open circuit point should be so situated.

In this arrangement the capacity current circulates in the pilot core C, the secondary windings B, B of the current transformers and the shield D, D as indicated by the arrows but does not afl ect the relays A, A.

In the arrangement shown in Figs. 3 and 4, A A are the relays which are of the balanced type having two windings, 13 B are the secondaries of the current transformers, and C the pilot core. The coinpensating conductors D D in this arrangement are in the form of companion cores to the pilot cores C as shown in Fig. 4c.

In this arrangement equal capacity currents will flow in the pilot core C, the companion core D D and the relays A and A but as the latter are balanced the eilect of the capacity currents in the relays is neutralized.

As shown in Fig. 3 the compensating conductors are severed or open circuited at points adjacent to the middle of the pilot cores.

The point should be midway between the current transformers B and B supposing those transformers to provide equal voltages but if, owing say to slight differences of manufacture, B gives a somewhat higher voltage than B then the point of open circuit should be proportionately farther from B and near B 2'. e. the part D of the compensating conductor should be longer than the part D so as to give equality of capacity current carried by C and D at one end and C and D at the other end.

Although the pilot cables are shown with only three working conductors, in certain cases four such conductors may be used the fourth conductor being employed to connect the neutral points of the relay and current transformer windings at either end.

The arrangements illustrated are intended for employment with tri-phase currents but it is to be understood that theinvention is not confined to three phase systems but may be used with single phase currents or systems with any number of phases.

If desired the pilot cables may be combined with telephone or power conductors.

What we claim as our invention and desire to secure by Letters Patent is 1. In protective means for electric distribution systems, the combination with pilot conducting means, of relay means and. current transformer means electrically connected in circuit with said pilot conducting means, and open circuited compensating conducting means adapted and arranged to neutralize the capacity current in said pilot conducting means.

2. In protective means for electric distri bution systems, the combination with pilot conducting means, of relay controlling means and secondary transformer means electrically connected in circuit with said pilot conducting means, and open-circuited compensating conducting means electrically;

connected in circuit with said pilot conducting means and arranged substantially parallel thereto to neutralize the capacity current in said pilot conducting means.

3. In protective means for electric distribution systems, the combination with multiphase pilot conducting means, of multiphase relay means and multi-phase current transformer means electrically connected in circuit with said pilot conducting means, and open-circuited compensating conducting means adapted and arranged to balance the capacity current in said pilot conducting means.

4:. In a protective arrangement for an electric distribution system the combination with a pilot wire of a companion conductor joined at each end to the pilot wire through a balanced relay which is connected to the secondary winding of a current transformer, the companion conductor being severed or open circuited at such a point as to give a balanced capacity current substantially as and for the purpose set forth.

5. In a protective arrangement for an electric distribution system, the combination with a pilot wire in electrical connection at each of its ends with the secondary winding of a current transformer and with a relay adapted to control a circuit-breaking device, of a compensating conductor having at .least one of its ends in electrical connection with the secondary winding of the transformer and the relay at the adjacent end of the pilot wire, such compensating conductor being severed or open-circuited so that no current can be transmitted through it, the arrangement being such that the relays are unaffected by the capacity current of the pilot wire.

6. In a protective arrangement for an electric distribution system, the combination of a pilot wire, a secondary winding, of a current transformer at each end of the pilot wire, and connected in series therewith, a relay in series with each secondary winding, said relay being adapted to control a switch or other circuit breaking device, and a compensating conductor in the form of a shield having at least one of its ends connected between the secondary windings and the relay at the adjacent end of the pilot wire, such compensating conductor being severed or open-circuited so that no current can be transmitted through it.

In testimony whereof we have signed our names to this specification in the presence of two subscribing witnesses.

PHILIP VASSAR HUNTER.

\Vitnesses ARTHUR JAMES HALL, R. BAGGA'IT.

JAMES ROBERT BEARD.

\Vitnesses F. H. IVILIJIAMs, S. A. STIGANT. 

